From the data recorded with the DELPHI detector at LEP in the years 1991-1992, 46 497 events were selected having a high-momentum muon in hadron jets. A fit to the average electric charge sum of the jets recoiling against a b-quark jet tagged by a high-PT muon results in an average mixing parameter of chiBAR = 0.144 +/- 0.014(stat.)+0.017/-0.011(syst.).